config: add a 'auto' value for the number of parallel jobs

When CT_PARALLEL_JOBS is -1, set the number of parallel jobs to the
number of online CPUs + 1. Update documentation to match.

I find this useful when building in the cloud. You can use the same
.config file and have the build adapt to the number of processors
available. Limited testing shows that NCPUS+1 is faster than NCPUS+0
or NCPUS+2.

Signed-off-by: Michael Hope <michael.hope@linaro.org>

 # Super-H specific configuration file

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_USE_MMU

 ## select ARCH_SUPPORTS_BOTH_ENDIAN

 ## select ARCH_DEFAULT_LE

 ##

 ## help The Super-H architecture, as defined by:

 ## help     http://www.renesas.com/fmwk.jsp?cnt=superh_family_landing.jsp&fp=/products/mpumcu/superh_family/